Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder can be operated using four cylinders. Cylinder deactivation improves fuel economy by reducing pumping losses. To smoothly transition between activated and deactivated modes, the internal combustion engine should produce torque with a minimum of disturbances. Otherwise, the transition will not be transparent to the driver. Excess torque causes engine surge and insufficient torque causes engine sag, both of which degrade the driving experience.
For an eight-cylinder engine, intake manifold pressure is significantly lower during eight-cylinder operation than during four-cylinder operation. During the transition from eight to four cylinders, there is a noticeable torque reduction or sagging in four-cylinder operation until the intake manifold reaches a proper manifold pressure level. In other words, there is less engine torque when cylinders are deactivated than when the cylinders are activated for the same accelerator position. The driver of the vehicle would be required to manually modulate the accelerator to provide compensation for the torque reduction and to smooth torque.
In commonly-owned U.S. patent application entitled “Engine Control System With Throttle Preload During Cylinder Deactivation”, Ser. No. 10/150,522, filed May 17, 2002, which is hereby incorporated by reference in its entirety, the throttle limit is adjusted to an increased position prior to cylinder deactivation to provide compensation. In “Spark Retard Control During Cylinder Transitions in a Displacement on Demand Engine”, Ser. No. 10/150,879 filed May 17, 2002, which is hereby incorporated by reference in its entirety, the increased throttle position or preload is accompanied by spark retard to offset torque increase caused by the preload before the cylinders are deactivated.